1. Field of the Invention
The present invention relates generally to a stationary plate rheometer; and more particularly, the invention provides a method and apparatus for measuring the melt viscoelastic properties of a material sample with a rheometer having a releasable stationary sample plate.
2. Description of Related Art
Testing for complete rheological data requires testing both the steady state and dynamic viscoelastic properties of a material sample. The rheological tests are performed to determine the molten viscoelastic properties of the material sample. Using a rheometer a material sample is placed between sample plates designed to contain the material, and the material sample is heated to its molten state.
The steady state viscoelastic properties are a measure of the material's shear properties, that is, the working properties of the material that will arise, for example, when molding material in a molten state. The dynamic viscoelastic properties are a measure of the intrinsic of properties of the material. To determine the steady state viscoelastic properties, one of the plates is held stationary; and the other plate is driven through a continuous rotation. The steady state viscoelastic properties are determined by measuring the resistance to rotation of the driven plate. The dynamic viscoelastic properties are determined in a similar manner in which a material sample is located between a stationary plate and a moveable plate. However, in this case, the moveable plate oscillates through a predetermined arc, and the dynamic viscoelastic properties are determined by measuring the resistance to rotation of the oscillating plate. Both the steady state test and the dynamic test may be performed on a single rheometer such as that disclosed in U.S. Pat. No. 4,539,838 issued to M. F. Fraleigh on Sep. 10, 1985.
Many commercially available cone and plate or parallel plate rheometers contain rotary drives which are switchable between a continuous rotation and an arcuate oscillation have a particular disadvantage, That being, when switching between the continuous rotation mode and the oscillation mode, the output shaft of the rotary drive mechanism experiences a rapid, erratic rotation that jerks or wrenches the driven moveable plate through an unanticipated incremental angular displacement or arc, of 60.degree. to 90.degree. relative to the stationary plate. At the end of the steady state test, the material sample may be worked such that it is highly viscous and presents a high resistance connection between the moveable and stationary plates. Therefore, the unanticipated rotation wrenching the moveable plate relative to the stationary plate when the rheometer is drive is changed from the continuous rotation to the arcuate oscillation may potentially damage the sample plates or the rheometer machine structure itself.
In addition, the unanticipated jerking or wrenching action provides an unanticipated working of the material sample which changes the rheological data, that is, the rheological characteristics of the material sample. Further, the magnitude of the change in rheological data is not known and unpredictable. Consequently, the rheological base line for subsequent testing of dynamic characteristics of the material sample are thus changed; and the rheological data obtained during the dynamic testing is skewed. Therefore, for the most accurate results, the material sample sheared in the steady state test should not be used for the dynamic test.